1. Field of the Invention
The present invention generally relates to a receiver, and more particularly, to a receiver for a weak radio wave.
2. Description of the Related Art
In receiving circuits used for a radio which receives a weak radio wave, such as a keyless entry system or simple data communications, a radio frequency (referred to as RF, hereinafter) is generally around 315 MHz or less. For example, when the RF is 315 MHz, an intermediate frequency (referred to as IF, hereinafter) of 455 kHz or 10.7 MHz is used in many cases. When the intermediate frequency is relatively high (i.e., 455 kHz or 10.7 MHz), desired characteristics have been realized by using and changing accessories such as a ceramic filter and a detection coil which are externally disposed from a semiconductor chip as an intermediate-frequency filter or a detection circuit.
However, there have been problems in that external accessories are large and expensive. The ceramic filter is used for the IF filter, and the detection coil is used for a phase shifting circuit of an FM detection circuit. There are ceramic filters with a narrow bandwidth (±5 kHz to ±15 kHz) for the IF frequency of 455 kHz and with a wide bandwidth (±50 kHz to 150 kHz) for the IF frequency of 10.7 MHz. Compared with a chip accessory, ceramic filters are much more expensive and the size thereof is also larger. The ceramic filter for the IF frequency 455 kHz is particularly large in size. While the height of the chip component is 1 mm or less, the height of a lead type ceramic filter is approximately 8 mm, and the height of a chip type ceramic filter is approximately 3 mm. In addition, the size of the detection coil is also large, and thus the conventional circuit has not been suitable for reducing costs and height.
On the other hand, when the intermediate frequency is as low as 100 kHz, the IF filter could be incorporated. In this case, an AGC (Automatic Gain Control) circuit may be connected to an output of the IF filter to detect an intermediate frequency signal and control the gain of an image rejection mixer so that an output signal of the IF filter is set to a predetermined level (refer to Japanese Patent Publication (Kokai) PH08-079125). Moreover, an adjustment circuit may be provided to set the center frequency of the IF filter equivalent to the intermediate frequency signal and the phase difference of the phase shifting circuit to 90° at the intermediate frequency (refer to Japanese Patent Publication (Kokai) PH08-070261). However, since it is difficult to change the bandwidth of the IF filter, which has band-pass filter characteristics, there has been a problem in that a semiconductor chip must be designed in accordance with its usage and performance, thus causing a lack of versatility.